Intravenous catheters come in basically two forms. First, there are catheters which are emplaced over the introducer needle. After emplacement of the catheter the needle is withdrawn from the center of the catheter and intravenous transfusion is possible. The second type are catheters commonly referred to as "through the needle" catheters. In these devices, the needle is emplaced into the vein with the catheter inserted inside the hollow needle. After insertion, the needle is withdrawn, usually by splitting the needle apart during withdraw. Thus, the catheter remains within the vein and is able to transfuse immediately upon emplacement of the needle.
Through the needle catheters have one very distinct advantage. That is, through the needle catheters are quite easy to insert into the vein because only the needle itself has to be inserted through body tissue. Whereas known needle making techniques afford easy insertion of the through the needle catheter, catheters themselves are somewhat more difficult to insert into the body when they are exposed around the needle. Thus, where possible, through the needle catheters have preference.
In contrast, through the needle catheters have two major disadvantages. Removal of the needle is difficult. Because the needle is generally configured to be smaller than the catheter hub, the catheter must be disassembled, or disconnected from the catheter hub so that removal of the needle may take place. Then, the catheter is reassembled to the catheter hub from which infusion was possible.
On the other hand, removal may be possible by sliding the needle over the hub connection. This method is less than desirable because the size of the needle increased. Or, the needle could be left in place and the catheter left to remain within the needle during infusion. This is undesirable because it causes reduced catheter size and left a foreign object within the body during infusion. This difficulty has been overcome through use of needles splittable along their diameter, and then removable over the catheter hub.
Second, since the catheter tubing must necessarily be smaller than the inner diameter of the needle in a through the needle catheter, the catheter itself presented problems. First, as previously discussed, the catheter maintained a low flow rate. The design of the needle necessarily limited the tube dimensions so that the inner diameter of the tubing became even smaller than the needle, causing such low rates of flow. On the other hand, because such tubing is smaller than the needle outside diameter, after needle removal the catheter may create a poor seal between the catheter outside diameter and the pierced body tissue. This results in leakage problems in the tissue outside the catheter.